1. Field of the Invention
This invention relates to a device and method for spreading and encoding of data signals. More specifically, it relates to a device and method for spreading and encoding of data signals by the CDMA method in wireless communication.
2. Description of the Related Art
Mobile communications are coming into widespread use. The communication methods that are used include FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access) and CDMA (Code Division Multiple Access). Among these, CDMA is attracting considerable attention as a standard next generation communication method.
In CDMA, by using a characteristic PN (Pseudo Noise) sequence for each user, it becomes possible for a plurality of users to share the same frequency simultaneously. On the sending side, the data to be transferred are multiplied by a PN sequence that has a faster frequency than the transferred data and then output. This spreads a bandwidth of the sending frequency band. That is to say, the spectrum is spread. On the receiving side, the transferred data are reproduced by multiplying the received data by a PN sequence that is the same as the PN sequence that was used on the sending side. The transferred data cannot be reproduced correctly by multiplying the received data by any PN sequence that is different from the PN sequence that was used on the sending side.
CDMA has the following characteristics. First, since a plurality of users share the same frequency at the same time, the number of user channels in a unit bandwidth can be increased. Second, since the sending frequency has a broad bandwidth, the effect of fading is slight. Third, since the data cannot be correctly reproduced unless the same PN sequence is used on the sending side and the receiving side, secrecy is maintained in the conversation.
However, since wireless frequency resources are (frequency bandwidth is) limited, as the number of mobile communications users increases, the number of channels in a unit bandwidth tends to increase. When the number of channels in a unit bandwidth thus increases, it becomes more difficult to correctly reproduce the transferred data on the receiving side. An error correcting code is frequently used to deal with this problem. An error correcting code adds a redundancy of the data to the data to be transferred.
Error correcting codes are sometimes used in CDMA. An existing encoder that provides an error correcting code in CDMA is shown in FIG. 1. The convolutional encoding section 501 is one form of error-correcting encoder. That is to say, the convolutional encoding section 501 provides redundancy to the data to be sent which are generated by the data generator 502. The convolutional encoding section 501 executes, for example, Hadamard encoding. In Hadamard code, the convolutional encoding section 501 has, for example, a shift register that stores the data to be sent while shifting the data one bit at a time, and a Walsh sequence generator that generates Walsh sequences. The spreading section 503 multiplies the output of the convolutional encoding section 501 by a PN sequence.
The error correcting code and the spreading code that are provided in CDMA are described in detail in references 1 and 2. In references 1 and 2, this method is called the LROCC (Low Rate Orthogonal Convolution Code).
Reference 1: Andrew J. Viterbi xe2x80x9cVery Low Rate Convolutional Codes for Maximum Theoretical Performance of Spread-Spectrum Multiple-Access Channelsxe2x80x9d, IEEE Journal on Selected Areas in Communications, Vol. 8, pp. 641-649 May 1990.
Reference 2: R. F. Ormondroyd and J. J. Maxey xe2x80x9cPerformance of Low-Rate Orthogonal Convolution Codes in DS-CDMA Applicationxe2x80x9d IEEE Transactions on Vehicular Technology, Vol. 46, pp. 320-328 May 1997.
When an error correcting code and spreading code are provided in CDMA, until now, as shown in FIG. 1, the section that executes error correcting encoding and the section that executes spreading have been independent of each other. For this reason, there is the problem that the scale of the circuitry becomes large.
As noted above, an error correcting code provides redundancy to data, so the amount of data that are actually transferred is increased, and the bandwidth is essentially spread. Consequently, even when the data are not multiplied by a PN sequence as in the example shown in FIG. 1, the data are spread. However, as discussed in reference 2, if only an existing type of error correcting encoder (in reference 2, a convolutional encoder equipped with an Hadamard encoder) is used and PN sequence multiplication processing is not executed, a peak is produced in the spectrum of data that are sent.
A peak in the spectrum causes the following problem. In CDMA, the secrecy of conversation is increased by making it appear as though the transferred data are noisy, but when a peak is produced in the spectrum, it becomes a source of interference, leading to a deterioration in the error rate. In addition, the equipment on the receiving side generally is configured so that an amplifier is used to amplify the received signal and then the desired signal is extracted; when a peak occurs in the spectrum, that peak is faithfully reproduced, making it necessary to have high priced equipment.
The purpose of this invention is to provide a device and a method to generate spread code data that will maintain good secrecy in conversation without increasing the scale of the circuitry or requiring high priced components.
The spread encoding device of this invention spreads data to be sent. It includes an m-bit shift register that stores the data to be sent, and a spreading unit for generating an M-sequence of which the period is 2mxe2x88x921, using the m bits of data to be sent that are stored in the shift register as an initial value, for generating an orthogonal M-sequence, and for outputting it as the spread sequence of the data to be sent.
In the configuration described above, the data to be sent are stored in the shift register sequentially, 1 bit at a time or several bits at a time. Then, every time new data to be sent are input into the shift register, the spreading unit generates an M-sequence using the m bits of data to be sent stored in that shift register as the initial value. Consequently, the data to be sent are spread by a factor of 2m at the same time that they are convolutionally encoded.
The spread encoding device of another embodiment of this invention includes an m-bit shift register that stores the data to be sent; a storing unit for storing a channel identifier that identifies a communication channel to which the data to be sent are assigned; and a spreading unit for generating an M sequence of period 2(m+n)xe2x88x921 with the m+n bits of data consisting of the m bits of data to be sent that are stored in the shift register and the n bit channel identifier that is stored in the storing unit as an initial value, for generating an orthogonal M sequence from that M sequence, and for outputting the orthogonal M sequence as the spread sequence of the data to be sent.
In the configuration described above, if the channel identifiers are different, then even if the m bits of data to be sent that are stored in the shift register are the same, a different orthogonal M-sequence will be generated. Consequently, the receiving device that receives the spread sequence that is generated by this spread encoding device can identify the user who sent the data.